1. Field of the Invention
The present invention relates to a semiconductor device having an external connection terminal formed with multi-layer unleaded metal plating, and a method of manufacturing the semiconductor device.
2. Description of the Related Art
Soldering is the widely used bonding method of electrically and mechanically connecting a semiconductor device to a printed circuit board.
In order to make connection with the solder easier, tin or tin-alloy plating is usually formed on the external connection terminal of the semiconductor device.
Most frequently used one among such alloy plating is tin-lead alloy plating, and the tin-lead alloy plating has excellent characteristics in functionality, quality, cost, etc.
Moreover, the soldering which is commonly used for the secondary mounting of the semiconductor device is also the tin-lead alloy plating, and the tin-lead alloy plating is congenial to the manufacture of the semiconductor device. Thus, the tin-lead alloy plating is conventionally used for the external connection terminals of the semiconductor device and it is excellent in view of the mounting characteristic and the reliability after the mounting.
In the process of tin-lead metal plating, the pre-treatment (etching, activation, etc.) of raw materials (iron-nickel alloy, copper alloy, etc.) is performed, and the plating of tin-lead alloy in the predetermined quantity is performed according to the electroplating method using the organic acid plating bath containing the metal ions of tin and lead after the pre-treatment of the raw materials.
Since tin and lead have the approximately same electrode potential, the control of the composition ratio of the tin-lead alloy can be easily performed. The metal ions of tin and lead can be supplied from the anode plate made of the tin-lead alloy, and it is possible to produce the tin-lead metal plating at low cost. Moreover, in view of the stability of metal plating, the ease of plating, and the cost of plating, the tin-lead alloy plating provides excellent features which are not seen in other tin alloy plating.
Japanese Laid-Open Patent Application No. 11-330340 discloses a semiconductor device having a lead (which is an electric wire) formed with multi-layer unleaded metal plating including the upper layer and the lower layer.
However, in recent years, from the viewpoint of earth environment protection, there is the outstanding anxiety about the influence of the toxicity of lead or lead alloys on the human body, especially the groundwater contamination. The restriction of the use of lead on the scale of the earth has been activated. Moreover, lead was listed as in the restriction of the use of hazardous substances (RoHS) by the Directive of the European Parliament and of the Council of Jan. 27, 2003, and it is urgently demanded to promote the unleaded production of electric devices.
In such circumstances, it is also demanded to expand the use of unleaded alloy plating on the external connection terminals of the semiconductor device, instead of the use of tin-lead alloy plating.
In the present situation, unleaded metal plating, including tin-bismuth alloy, tin-silver alloy, tin-copper alloy, etc. is under development and put in practical use. Furthermore, other unleaded metal plating, including tin-zinc alloy, tin-indium alloy, etc. are considered as candidates for the unleaded metal plating.
In addition, 100% tin metal plating is not exactly the alloy plating but it is also a candidate for the unleaded metal plating.
The 100% tin metal plating is already put in practical use but it has the problem of whisker resistance left behind. As the advantageous feature of the pure tin plating, the metal-plating processing can be easily performed at low cost. In view of this point, it can be said that it is advantageous beyond the tin-lead alloy plating, from the past actual results and from the fact that 100% tin metal plating is not the tin alloy plating.
However, as long as whisker resistance is concerned, there is no secondary element in the 100% tin metal plating which should be suppressed, and it is a metal plating with which whisker tends to occur. The time of the occurrence of whisker and the scale of whisker after the products from the factory are delivered to the market cannot be predicted. There is the possibility that it becomes a serious problem.
Recently, there have been proposed some kinds of the 100% tin plating with which whisker does not easily occur. However, the occurrence mechanism of whisker is not solved completely, but they are merely the results of the experiments and still cannot be put in practical use. For this reason, it is necessary to take the great risk for adopting the 100% tin plating.
What are mainly put in practical use as the current unleaded metal plating for the time being are the tin-bismuth alloy, the tin-silver alloy, and the tin-copper alloy. These unleaded metal alloys have some merits and some demerits, and there is none of them that is equivalent to the tin-lead alloy plating in all the viewpoints of the quality of the metal-plating coat, the stability of the metal-plating processing, the cost, etc., respectively.
As the problems of the current unleaded metal plating candidates mentioned above, the merits and the demerit of each of the respective unleaded metal plating candidates will be explained.
First, the tin-bismuth alloy metal plating has the comparatively omnipotent character, and its proliferation level is the highest among the unleaded alloy plating candidates for the semiconductor device.
The shortcoming of the tin-bismuth alloy metal plating is the characteristics of bismuth which makes this alloy metal hard and brittle, and it causes the occurrence of cracks of the metal-plating coat due to the bending of the external terminals of the semiconductor device as well as the lowering of the solder bonding strength after the secondary mounting. In addition, the inclusion of bismuth in the alloy metal plating makes the solder wettability fall, which is also one of the concern items.
It is known that the tin-bismuth alloy metal plating has a high whisker resistance as compared with other unleaded metal plating. However, because of the above-mentioned shortcoming, it is difficult to raise the content ratio of bismuth of the tin-bismuth alloy metal plating so much. The actual condition is that the level of whisker resistance which is equivalent to the level of the tin-lead alloy plating is not secured by the current tin-bismuth alloy metal plating. Although the tin-bismuth alloy metal plating has some problems, they are comparatively minor defects, and the quality which can be equivalent to the practical-use level is secured mostly.
Next, the tin-silver alloy metal plating has a comparatively good reliability after the secondary mounting, and it is the advantage that the tin-silver alloy metal plating is congenial to the tin-silver alloy which is the current unleaded secondary mounting solder.
The shortcoming of the tin-silver alloy metal plating is that the metal-plating coat is easy to corrode, which causes the problems, such as discoloration of the metal plating and migration. Furthermore, the tin-silver alloy metal plating has also some problems that its whisker resistance is inferior to that of the tin-bismuth alloy metal plating, and that the solder wettability falls due to the inclusion of silver therein.
Moreover, also except functionality, the stability of the metal-plating liquid is poor, which causes the problem that the running cost including the metal-plating liquid adjustment etc. becomes very high.
The environment where the tin-silver alloy metal plating can be used is limited, and the cost becomes high. However, since the reliability after the secondary mounting is comparatively good, the tin-silver alloy metal plating is used in many cases in the environment where the tin-bismuth alloy metal plating cannot be used.
Finally, the tin-copper alloy metal plating is comparatively easy to use, and the cost is comparatively low. But the shortcoming of the tin-copper alloy metal plating is that it has a low whisker resistance and a poor solder wettability. Especially, the environment where the tin-copper alloy metal plating can be used is limited.
Furthermore, there is also the problem in that any technique which measures the metal-plating coat composition and thickness on the copper material in non-destructive manner is not established. Moreover, although it is comparatively easy to use, the composition of copper which becomes eutectic in tin-copper is 1% or less, and it is necessary to set the content ratio of copper as low as possible when the influence on the solder wettability is taken into consideration.
For this reason, the content ratio of copper in the order of 1% will be set up in the actual metal-plating process. However, the numerical value is extremely low, the management of the content ratio of copper is very difficult. In addition, it is the limit also as measuring instrument accuracy, and managing as the product warranties is hardly carried out for the practical use.
Moreover, in the latest report, degradation of the tin-copper plating liquid is severe, and the adjustment by the supplement cannot be performed. As long as the cost in the long run is concerned, it cannot be said that the expense is cheap.
On the other hand, in the tin-bismuth alloy metal plating method disclosed in Japanese Laid-Open Patent Application No. 11-330340, the bismuth content of the lower layer in the tin-bismuth alloy metal plating is low, and the bismuth content of the upper layer is high. It is difficult to overcome the problems, such as the occurrence of cracks of the metal-plating coat due to the bending of the external terminals of the semiconductor device as well as the lowering of the solder bonding strength after the secondary mounting.